Modern imaging devices use electronic arrays to capture images. The arrays have pixels that generate electric charges, such as electrons, when they are exposed to light from an image. The generated charges of each pixel are stored and then read out, for rendering the image.
It is possible for imaging devices in the prior art to detect movement. However, such detection often requires that that the pixels image twice, in other words, integrate over two frames, and then that the two frames be contrasted. Such consumes a lot of power.
In other instances, detection of movement requires that the object be illuminated with a modulated light source. Light sources consume power, while it is desired to preserve power within the imaging device.